I have this memory, where I was exploring PH in the 90s and there was this invisible trapdoor opening beneath your legs and you dropped to the main room and died. Its not in a DFU. But I cant see it in the vanila as well. It drives me crazy. Was it only in the demo?

I did not play video games when I was a child, only started as a teen (saw my first computer at 11), but I had memories of movies and spent some time trying to find what they were (because I remembered scenes, not titles): The Omega Man (1971, scifi/horror vampire movie), L'invenzione di Morel (somewhat obscure italian tv movie from 1974, low budget scifi but mind blowing),... and each time they turn out to be quite different from what I remembered.

So I'm not surprized in the least, it could very well be that you conflated several dungeons in one. If so that's great, I like this memory

When a measure becomes a target, it ceases to be a good measure.
-- Charles Goodhart

I think there are a few dungeon blocks that do have those kind of trapdoors but I don't think PH has one. I will check the dungeon blocks sometime when I feel like it to identify which ones have a trapdoor. I want to figure out how to set those up anyway